Changes in v3

Although v2 of this library worked fine for most uses, there were some issues with the architecture, and it lacked a proper test suite. v3, which is coming soon, adds in unit and integration tests, squashes some bugs, adds .NET Standard 2.0 compatibility, and makes some other significant changes. Many of these are breaking changes, although since the overall design remains the same, it shouldn't be very complicated to transition. Among the visible changes are:

• All library options are now fully async. Non-async methods have been removed.
• The file format for saved configuration has been changed from XML to JSON.
• MailmanClient used to be a descendant of RestClient and made exposed all of that class's public properties. It now has a RestClient as a private field and exposes (and delegates) only certain properties, like Authenticator and Proxy.